1. Field of the Invention
The invention relates to a movable calibration device for a sheet-fed scanner, and more particularly to a calibration device including a calibration sheet driven by a cam for the calibration of the sheet-fed scanner.
2. Description of the Related Art
FIGS. 1 and 2 are schematic illustrations showing a prior art calibration device for a sheet-fed scanner. In this calibration device, as shown in FIGS. 1 and 2, a motor 114 rotates a cylinder 112, around which a calibration sheet 116 is wound. When the cylinder 112 is rotated clockwise, the calibration sheet 116 is unrolled and spread out underneath a scanning module 110. When the cylinder 112 is rotated counterclockwise, the calibration sheet 116 is retracted away from the scanning module 110. In order to prevent accumulation of errors during the process, a detector 118 has to be provided to detect whether or not the calibration sheet 116 has been moved away from the scanning module 110.
The conventional calibration utilizes a detector, a reversible motor and a gear set, converting the rotational motion of the cylinder 112 into linear motion, such that the calibration sheet made of Mylar may be moved into the scan window of the scanning module and then away from the scan window after the calibration is completed.
However, because Mylar tends to yield to weight and pressure, the rotational motion cannot be completely converted to the linear motion. Thus, the scanning module may not correctly focus on the calibration sheet, and the results are thus affected. In addition, the structure of this calibration device with the detector and the reversible motor is complicated, and the cost cannot be effectively reduced.